Conventional electrochemical fuel cells convert fuel and oxidant, generally both in the form of gaseous streams, into electrical energy and a reaction product. A common type of electrochemical fuel cell for reacting hydrogen and oxygen comprises a polymeric ion transfer membrane, also known as a proton exchange membrane (PEM), within a membrane-electrode assembly (MEA), with fuel and air being passed over respective sides of the membrane. Protons (i.e. hydrogen ions) are conducted through the membrane, balanced by electrons conducted through a circuit connecting the anode and cathode of the fuel cell. To increase the available voltage, a stack is formed comprising a number of series-connected MEAs arranged with separate anode and cathode fluid flow paths. Such a stack is typically in the form of a block comprising numerous individual fuel cell plates held together by end plates at either end of the stack.
Because the reaction of fuel and oxidant generates heat as well as electrical power, a fuel cell stack requires cooling once an operating temperature has been reached, to avoid damage to the fuel cells. Cooling may be achieved by forcing air through the fuel cell stack. In an open cathode stack, the oxidant flow path and the coolant flow path are the same, i.e. forcing air through the cathode fluid flow paths both supplies oxidant to the cathodes and cools the stack.
However, optimal operation of the fuel cell stack relies on maintaining the fuel cells at their optimal operating temperature and fuel cell stack efficiency can be adversely affected at low ambient temperatures or when a stack is starting up from cold. Thus, it is desirable to be able to regulate the cooling efficiency of air flows through the cathode.
One technique for achieving this is to recycle some or all of the exhaust air from a fuel cell stack that has been passed over the cathodes back to the stack air input. The exhaust air is preheated by its first passage through the stack, and a duct takes this exhaust air around to the front of the stack to re-use, possibly mixed with a proportion of cool air, thus reducing the overall cooling efficiency and allowing the fuel cell stack to run efficiently at low ambient temperatures. A potential disadvantage of this arrangement is that extensive ducting is required to pass air from an output face of the fuel cell stack, right around the stack to the input face. This adds to the bulk of the fuel cell system and limits the amount of space for other support systems to be built on to the fuel cell stack.
A further potential disadvantage of this recirculating arrangement is that the recirculated warm air, when mixed with very cold ambient air, can cause substantial condensation to occur at the inlet to the fuel cell stack.